Satellite-Based Auto-Guidance
Auto-steer technology has taken the farming community by storm in recent years. This technology is more frequenlty referred to as auto-guidance, the guidance of agricultural vehicles using satellite-based positioning equipment (e.g., GPS receivers).
Rising energy costs and more reasonably priced auto-guidance systems have made a clearer cost justification for investment in this new technology. As many of the benefits of auto-guidance technology become increasingly evident, early adopters continue discovering additional advantages. The most obvious rewards include:
- Reduced skips and overlaps
- Lower operator fatigue
- Ability to work in poor visibility conditions
- Minimal setup and service time
- Ease of use
The three levels of automation for steering an agricultural vehicle:
- Navigation aids - Relatively inexpensive navigation aids, known as parallel tracking devices or, more commonly, lightbars, are being used by operators to visualize their position with respect to previous passes and to recognize the need to make steering adjustments if a measured geographic position deviates from the desired track.
- Auto-guidance - More advanced auto-guidance options include similar capabilities with the additional option of automatically steering the vehicle using either an integrated electro-hydraulic control system or a mechanical steering device installed inside the cab. When implementing an auto-guidance option, the operator takes control during turns and other maneuvers and over-sees equipment performance when the auto-guidance mode is engaged.
- Field robots - Finally, with autonomous vehicles, the operator’s presence on board is not required and the entire operation is controlled remotely (via wireless communication) or in robotic mode. This can be beneficial, for example, when applying chemicals that are hazardous to human health. The greatest liability of autonomous vehicles, improper response in unpredictable field situations, has been the major drawback of robotic agriculture. Therefore, auto-guidance has been recognized as the most promising option for today’s farming operations.
Despite the type of system used, since the radio signal processed by receivers can be affected by several factors (atmospheric interference, configuration of satellites in the sky, time estimation uncertainties, etc.), the applicability of uncorrected position estimates is rather limited. To adjust estimated geographic coordinates in real time, various differential correction services are used. In addition to the differential correction, most receivers apply signal filtering techniques to assure the best possible predictability of antenna location. Based on the quality of differential correction and internal signal processing, positioning receivers used for auto-guidance have been advertised according to the level of anticipated accuracy: sub-meter, decimeter, and centimeter.
Positioning receiver accuracy Levels:
- Sub-meter - Widely used in agriculture and other industries, single-frequency receivers with submeter level accuracy frequently rely on several alternative differential correction services provided by public and private entities. Popular in the past, the Coast Guard differential correction AM radio signal (known more commonly as Beacon) is broadcast through a network of towers located near navigable waters. More recently, Wide Area Augmentation System (WAAS) has been deployed by the Federal Aviation Administration to broadcast a satellite-based differential correction service. A similar service is available through free-of-charge John Deere StarFire 1 (SF1) and subscription-based OmniSTAR Virtual Base Station (VBS) options.
- Decimeter - To achieve decimeter level accuracy, dual-frequency receivers can be used with subscription-based John Deere StarFire2 (SF2) or correction services, or with a local base DGPS station.
- Centimeter - A local base station is also required to implement a Real Time Kinematic (RTK) differential correction service, which provides the ultimate centimeter level of accuracy. In certain locations around the US, local networks of permanent RTK base stations have been established by private entities to provide fee-based coverage of areas with relatively high demand for superior positioning accuracy.
Overall Performance
When adapting auto-guidance to a particular farm operation, it is necessary to understand that positioning error is just one factor causing less than perfect field performance. In addition, the ability to maintain desirable geometric relationships between passes is affected by vehicle dynamics, ability of the field implement to track behind the vehicle, and actual conditions of the field surface. Therefore, poor quality of the steering control system, sloped terrain, or misalignments in the implement will cause the overall field performance to suffer.
Currently, hands-free steering of agricultural vehicles is accomplished using either a steering device attached to the steering column or through an electro-hydraulic steering system. An easy-to-setup steering column device can be attached to an existing steering wheel or the steering wheel can be replaced with an actuator module that includes its own steering wheel. Auto-guidance systems integrated with electro-hydraulic steering control circuits alter the travel direction similar to conventional power steering. A control valve is used to properly direct hydraulic oil when a steering adjustment needs to be made. When retrofitting old tractors some manufacturers provide other hydraulic drive components to guarantee the required steering performance. It is obvious that actuators adjusting direction of travel through a steering column can be less responsive than those that change the orientation of vehicle wheels directly. In most instances, a wheel angle sensor is used as a steering feedback in addition to the records of heading obtained from the GPS receiver. This makes electro-hydraulic steering systems even more reliable.
Control of vehicle dynamics becomes more challenging when farming sloped ground. Thus, roll (tilt from side to side), pitch (tilt from front to back) and yaw (turn around vertical axis) alter location of the positioning antenna with respect to other parts of the vehicle. For example, when driving along a slope, the horizontal position of the antenna located on the top of a cab shifts to one side of the tractor with respect to the projected center of the tractor. This causes an engaged steering control system to guide the vehicle so that the point directly below the antenna (not the center of the vehicle) would follow the desired pass. To compensate for these attitude-caused challenges, most auto-guidance systems include a combination of gyroscopes and accelerometers or several antennas placed in different locations on the cab. Less advanced terrain compensation modules can deal only with roll and pitch angles, while more sophisticated sensing systems, frequently called 6-axes, can measure the total dynamic attitude of the vehicle in space.
Vehicle stability and proper alignment of the implement attached to the vehicle are also important when implementing auto-guidance. If a skip followed by an overlap takes place with every alternating pass in the opposite direction when making straight and level trips from one end of the field to the other, even a properly adjusted pulled implement will not follow the on sloped terrain. In that case the implement will tend to stay close to the center of a turn or shift downward.
Several manufacturers have addressed implement tracking solutions that allow accurate sensing of the implement’s position with respect to the vehicle and mechanical adjustment of this position using a set of large-diameter disc coulters to overcome the occurring side shift. Additional developments are focused on compensating for known shifts of the implement by adjusting the vehicle’s trajectory to assure proper tracking of the implement instead of the vehicle. Optical and mechanical crop-based guidance systems can also be useful when it comes to the position of the implement with respect to previously established rows.
Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by University of Nebraska–Lincoln Extension is implied.
For additional informaton, see UNL Extension Circular, Satellite-Based Auto-Guidance - EC 706 (873 KB; 6 pages)
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